The apparatuses chiefly used for the concentration of liquid mixtures include multiple-effect concentrators and vacuum concentrators.
The first type of technology comprises a series arrangement of two or more concentrators each comprising a container filled with the mixture to be concentrated, a coil heating device which is immersed in the liquid mixture and which is generally fed with steam, and finally a pipe system which connects in series the two or more containers constituting the plant. The liquid mixture in the first container is heated and concentrated by means of mains steam. After having reached a specific degree of concentration, the liquid mixture is conveyed into the next container where it is subjected to an analogous treatment, the only difference being that, in this case, the mains steam is replaced by the vapour coming from the first container, or that released from the liquid mixture in the first concentration stage. The process just described can be repeated several times until the desired concentration is obtained.
With this type of technology, however, it is possible to reach only a specific degree of concentration, which is determined by the viscosity of the product to be treated, because it is necessary to ensure that the product, which becomes gradually more dense, runs well from one container to another in the concentration plant in order to avoid undesired obstructions in the connecting pipe system. This problem is further aggravated if the liquid mixture to be treated contains various types of fibre, or aggregates of insoluble salts or the like. A further disadvantage presented by any type of liquid mixture to be treated, however, is that of the encrustations which form on the heating coils which have to be dismounted and cleaned periodically.
The second type of apparatus, the vacuum concentrators, normally comprise a container heated by a jacket or a heating coil, the latter being immersed in the liquid mixture to be treated, and a condenser for condensing the vapour formed in the concentration stage.
This type of apparatus, however, has the disadvantage of operating batchwise which, as is well known, limits productivity and requires more complex management operations. Added to this are the above-mentioned problems of the encrustations on the coils or other parts of the plant, and also a considerable waste of energy owing to the maintenance of reduced pressure in the plant.